TWDM ONU (time division multiplexing optical network unit) is one of the technical difficulties of NGPON (Next Generation Passive Optical Network), which is mainly due to the fact that DWDM FILTER (dense wavelength-division multiplexing filter) restricts that the bandwidth of each wavelength channel to be very narrow. For example, for each channel, the DWDM FILTER passband is actually within the range of channel center wavelength±80 pm. In this case, for a normal ONU, under the control of the system signal BEN, the module's laser continuously switches between the light emitting and non-emitting states at any time. As a result, the temperature of the laser is going to change at any time point. On the other hand, the temperature of the laser has a corresponding relationship with the wavelength thereof. Under normal circumstances, when the temperature of a laser increases by 1° C., the wavelength of the laser will increase by 0.1 nm; while when the temperature of a laser decreases by 1° C., the wavelength of the laser will decrease by 0.1 nm. Accordingly, it is very difficult to control the wavelength. In the existing technologies, a software PID is utilized to adjust the current temperature to make it reach a target temperature. However, such software PID adjustment has the issues of slow sampling, slow responding. Thus, the temperature cannot be accurately adjusted, causing the wavelength deviation to exceed the range of the DWDM passband.